With the advent of science and engineering on the nanometer scale, scanning probe microscopes are increasingly used for nanometrology applications. Scanning probe microscopes use various probe tip configurations for assessing a range of physical properties on the atomic and nanometer scale. Based on the physical detection principle, a scanning probe microscope can be referred to as a scanning tunneling microscope, an atomic force microscope, a magnetic force microscope, or other type of microscope. The availability of these various configurations has permitted a wide range of nanometer scale measurements.
One particular application of scanning probe microscopy is in the measurement and characterization of specimen topography. However, accurate specimen metrology requires dimensional standards and standardized procedures for calibration. An absolute length reference is typically unavailable on a sample of interest, and nanometrology device standards are needed. Conventional “top-down” approaches for fabricating length references are based on conventional semiconductor manufacturing lithographic and etching processes. However, these processes are suitable for reference standards having feature heights greater than about 20 nm and feature pitches greater than about 1 μm, and smaller feature sizes are unavailable.
In many applications, interactions of a scanning probe tip and a sample are complex, and measurements of sample topography are complicated by the finite size of the scanning probe tip. See, for example, J. G. Griffith and D. A. Grigg, “Dimensional metrology with scanning probe microscopes,” J. Appl. Phys. 74, R83 (1993) and D. J. Keller and F. S. Franke, “Envelope reconstruction of probe microscope images,” Surface Science 294, 409 (1993). Accurate microscope calibration could permit deconvolution of the effects of finite probe tip size from specimen measurements.
FIGS. 1, 2, and 3A-3B illustrate a typical lithographically produced nanometrology device standard and microscope calibration based on such a device standard. Using such a device, a microscope can be calibrated for lateral dimensional measurements, and lateral non-linearity, hysteresis, creep, and cross-coupling effects can be identified and compensated. FIGS. 1-2 illustrate a conventional silicon calibration grating that includes a chessboard-like array of square pillars with sharp undercut edges formed by (110) planes of silicon. Step heights are on the order of 1 μm and the pitch is on the order of a few μm, wherein the pitch is generally accurate to within ±7.5 nm. The edge curvature is less than 5 nm. FIGS. 3A-3B illustrate scanning probe microscopy images before and after lateral scan nonlinearities are corrected, respectively, based on a nanometrology standard such as that illustrated in FIGS. 1-2.
FIGS. 4-5 illustrate a conventional “top-down” nanometrology standard for vertical calibrations. This standard includes a quasi-one-dimensional array of rectangular SiO2 steps that are 20 to 25 nm high and that are formed on a silicon wafer. The area of this standard is about 9 mm2. Step height is typically accurate to about ±1 nm. The array is generally coated with Si3N4 to prevent Si from oxidation. Vertical calibration and linearity corrections are typically based on several such devices with different nominal step heights such as, for example, 22.5 nm, 100 nm, 500 nm, 1000 nm, and 1500 nm.
Since scanning tunneling microscopes (and special atomic force microscopes that operate in vacuum) can have resolution on an atomic scale, reference samples of inorganic crystalline materials having known lattice constants can be used as lateral calibration standards. For example, highly ordered pyrolytic graphite can be used. In a [0001] oriented graphite crystal, the lateral lattice constant, i.e. the length of the <1000> and <1100> lattice vectors can be measured directly and is known to be 0.2462 nm. A single atomic monolayer step of graphite is known to be 0.3354 nm high. Thus, multilayer steps must have heights that are integral multiples of this monolayer step height. Such single- and multilayer steps can be used for vertical calibrations of scanning probe microscopes. Standards based on other inorganic crystals can also be used to obtain lateral and vertical calibrations of less than about 1 nm. For example, a Si (111) surface such as illustrated in FIG. 6 can be used. This surface has a lateral period of 2.688 nm.
Thus, nanometrology device standards can be obtained that are suitable for calibrations from atomic dimensions up to a few nm. There is, however, a gap between these atomic precision nanometrology device standards and the about 20 nm dimensions of nanometrology device standards that are produced by the top-down approach. Accordingly, improved device standards and methods are needed.